Ultrasonic inspection consists of subjecting a structure to ultrasonic signals, i.e. signals having a frequency above approximately 20 kHz up to approximately 25 MHz, and receiving ultrasonic signals transmitted or reflected by said structure. The transmission or reflection of ultrasonic signals by a material provides information about its structural properties, such as thickness and flaws. Flaws, such as voids, inclusions and delaminations in case of a laminated structure, are areas with different ultrasonic transmission properties. Accordingly, transmitted or reflected signals from an area of a structure having a defect differ from the signals transmitted or reflected from areas of the structure having no defect.
British patent application 2,204,690 discloses an ultrasonic method of testing the strength of a bonded joint by mutually comparing signals received from different areas of a structure under inspection. In order to separate the transmitted signals from the reflected signals or to filter out all reflected signals from positions or interfaces other than those from the joint under examination, the received signals are subjected to a first time-gating operation. The signals are subjected to a further time-gating operation for smoothening and integration purposes.
During the ultrasonic inspection of panels of ARALL (trademark), some remarkable phenomena occurred. ARALL is a laminated material consisting of relatively thin aluminum layers bonded together with a fibre reinforced resin. The material is usually fabricated in flat panels with two to six aluminum layers and one to five fibre reinforced resin layers, in particular aramide fibres, in the present specification called fibre layers. During production of the material, several types of defects can arise, the most important of which are delaminations, porous bond lines and unintended inclusions within the bond lines.
When subjecting a sample of ARALL to ultrasonic signals, areas with a relatively low transmission coefficient at one frequency showed an increased transmission at another frequency. Furthermore, areas were found with reduced transmission properties while flaws such as voids, delaminations and inclusions were absent, following microscopic examination of said areas. It was found that these effects appeared to be frequency related.
After studying the transmission and reflection properties of ARALL in the frequency domain, it was found that only within certain frequency bands ultrasound is nearly unattenuated transmitted, whereas it is totally or partly reflected in other frequency bands. This behavior can be explained in that ultrasonic signals transmitted by one layer or a combination of layers can be reflected by other layers. Multiple reflections in one layer or a combination of layers can give rise to destructive and to constructive interferences, leading to transmission and reflection bands.
Thus a multi-layer structure has been found to have alternating ultrasonic transmission frequency bands comprising relatively higher transmissivity frequency bands having a low reflectivity separated by relatively lower transmissivity frequency bands having a high reflectivity. The term "transmission band" is used herein to refer to such a higher transmissivity ultrasonic frequency band.
Physically, ARALL can be regarded as a complex (acoustical) filter. The order of the filter increases with the number of layers. As long as the layer parameters, such as layer thicknesses, the densities of the layer materials and the acoustic velocity in each layer are exactly equal among all aluminum and among all fibre layers, the filter behaves in a regular manner; i.e. the position of transmission and reflection frequency bands is determined by the actual parameter values (layer thickness and density, etc).
However, when one or more of said layer parameters vary from layer to layer for a given location, the frequency characteristics of the structure become more complex.
Accordingly, apart from attenuation or scattering due to defects such as voids, inclusions and delaminations, variations in the layer parameters of the ARALL laminates influence the transmitted and reflected signals, with the risk of misinterpretation of such transmitted and reflected signals as being caused by voids, delaminations, inclusions, etc. in the laminate.